Written in the Rings
By Clara Wodny on January 28, 2025 in Blog

When you think of Eastern redcedars (Juniperus virginiana), what feelings come to mind? Many, especially those with prairies or pastures to manage, associate the species with a sense of annoyance and frustration due to their tendency to quickly spread into areas where they are not wanted, crowding out other species in the process. However, these trees have incredible stories to tell if we’re willing to shift perspectives and view them with the awe and respect they deserve.
Every native species, including redcedar, has an ecological niche. That is, they have evolved to thrive under certain conditions and fill a specific role in the ecosystem. Despite their bad reputation, Eastern redcedars have been here all along — they are among the oldest living things in Iowa. Before labeling them as unequivocally “invasive,” it is important to understand how our changing land stewardship philosophies and practices have affected the natural world.
Eastern redcedars are incredibly tolerant of a wide variety of growing environments, including extremes of drought, temperature, wind and soil conditions, making them capable of living in some of the harshest places on the landscape. In short, they are — and always have been — able to grow almost anywhere.
In contrast to oaks, redcedars have thin bark that provides little insulation to the living tissue and wide spreading branches that make them susceptible to fire. Historically, redcedars have thrived on cliffsides, riverbanks and rocky bluffs — places where they were sheltered from passing prairie fires. However, while the landscape used to be covered in prairie and maintained with fire, it is now largely maintained by plows and machinery. Because they are no longer being culled by fire, the trees spread into more open areas, like prairies and pastures, and serve as a visible reminder of the interconnected impacts of our choices when it comes to land management.
“We’ve spent a long time thinking that the social and environmental systems of our world are separate,” says Evan Larson, a professor and chair in the University of Wisconsin-Platteville’s Department of Environmental Sciences & Society. “And it’s becoming very clear that simply is not the case.”
Lucky for us, redcedars are steadfast observers that physically respond to their environment, storing accurate and detailed records of their experiences in their trunks. This information is encoded as tree rings, with each ring representing one particular year. The science of dating trees and interpreting past events based on the analysis of tree rings is called dendrochronology.
Stories are written into tree rings in their own language, and dendrochronologists are skilled translators. This is certainly true of Evan Larson, his colleague James Riser and their team of undergraduate students at the University of Wisconsin-Platteville, who are currently engaged in a three-year study dedicated to expanding understanding of past drought and rain patterns across Iowa and the Great Plains. This study aims to produce a Great Plains drought reconstruction that spans the past 1,000 years, filling a noticeable gap in previous research. Funded by the National Science Foundation, their research is headquartered at UW-Platteville’s Tree-Ring, Earth, and Environmental Sciences Laboratory (TREES Lab). Projects completed through the TREES Lab are specifically designed to provide opportunities for undergraduate students to engage in authentic, meaningful research as part of their education.
Before beginning to analyze tree-ring data, dendrochronologists need to obtain core samples from both living and dead redcedars in the region. When choosing which trees to sample, they seek out the ones that have lived for several hundred years and are able to provide the longest record. If you assume that means they’re looking for the biggest, tallest trees, you’re in for a surprise.
“I don’t care about the big, straight trees,” says Riser. “It’s the gnarly, twisted, weathered ones that have an amazing record and story to tell, compared to younger trees with wide rings that haven’t experienced as much hardship.”
When working with living trees, dendrochronologists use a special tool called an increment borer to extract a core sample — about the size of a pencil — that will cause no harm to the tree. Trees that are already dead don’t require as much care, as full medallions are able to be extracted from the trunk with a saw. Still, the researchers remain conscious of protecting the landscape and do their best to source from areas that will contain the most information in the least obtrusive way.
So, where do they find these trees?
At one point in time, most stony outcrops, overlooks and bluffs would have supported a pocket of old eastern redcedar trees. On Iowa’s prairie landscapes, trees were relatively rare — and timber resources so valuable — that nearly everything was cut for use. As a result, most bluff sites today host living cedars that have only sprung up in the last 150 years. However, not all is lost — these younger trees shade old, gray, weathered stumps that persist as reminders of the trees that lived there before.
Many of these stumps were already hundreds of years old when they were cut down in the 1800s. While these stumps are still able to provide valuable data, they present distinct challenges to dendrochronologists: how do they determine when the stumps were cut? How can the rings of long-dead trees provide information about the complex relationship between climate and tree growth? How can the memories of these trees, now stumps that contain hundreds of rings, be translated into a more complete understanding of our shared climate history?
One particular site included in this study, Heritage Valley, holds the key to answering many of these questions: truly ancient, still living redcedars.
Heritage Valley, a nearly 1,200-acre property in Allamakee County, was purchased by INHF in 2007 with the assistance of over a thousand donors. Heritage Valley boasts some of the oldest trees in the area — the ancient ridge-top trees provide rare examples of living trees whose lives span 450 years or more, which means they can provide the necessary connection between this generation and the generation of stumps found at many other sites.
“There is no good reason to think you’ll find trees in Iowa older than this,” says Riser.
The hillsides at Heritage Valley face southwest and are elevated far above the moisture of the river valley, creating dry and exposed conditions. This causes the trees to be extremely sensitive to even subtle changes in precipitation and temperature, making them ideal recorders of the site’s climate history as they feel and embody years of harsh conditions and drought more clearly.
Once a tree has been sampled, the team is able to visually estimate how many rings are present and get an idea of the type of data it will provide. Still, there is always room for surprises during the process of analysis.
When samples enter the lab, they must be sanded to a high polish with 400-600 grit sandpaper before the official dating and analysis can begin. This often reveals exciting treasures, such as unexpected tree-rings that are mere fractions of a millimeter in width.
Larson recalls early on in this project when they took several samples at Turkey River Mounds State Preserve along the Mississippi; though satisfied, Larson and Riser were slightly underwhelmed. Visual estimations in the field indicated the best sample only contained about 200 rings. The samples sat in the lab, unprepared, for months until a particularly ambitious student decided to sand each of them. The sample with the estimated 200 rings ended up including over 470 rings, dating all the way back to the 1300s.
You might be wondering how exactly they know that the tree was alive in the 1300s, despite being dead for over a century. This is determined using a process called cross-dating.
The fact that tree growth is so heavily intertwined with environmental conditions means that you can tell exactly what kind of conditions a tree experienced in any given year by looking at the size and colors of the ring. In most cases, trees of a particular species in the same region will experience similar climate conditions, therefore forming rings that are comparable in width to their neighbors each year. As conditions change from year to year, so too do the width of tree rings across a region.
Given enough time, these year-to-year changes create a pattern that is as distinct as a fingerprint. By matching this temporal fingerprint from the samples collected from dead trees to those of living trees, such as the ancient redcedars at Heritage Valley, dendrochronologists can build a timeline. This comparison and cross-referencing across samples enables Larson, Riser and their students to assign the exact year of formation to each ring in each sample with absolute precision.
The cross-dating process begins with the bark of living trees, which represents the current year. Working backwards from the bark, dendrochronologists can then identify and count the rings, assigning each one to a specific year. Once this original timeline is established, they are then able to take a piece of deadwood, match up its rings with some of the living tree rings, and assign them to a year as well. After determining where the dead wood overlaps with the living wood, dendrochronologists can keep working backwards, stretching the chronology further and further back in time.
Rings are defined by the color of the wood. At the beginning of each year, trees grow lighter colored earlywood made of bigger cells with thinner cell walls. As tree growth slows down and the tree starts to prepare for dormancy, the wood becomes gradually darker, with smaller cells and thicker cell walls packed closer together. The actual period of dormancy, during winter, is an invisible line between the smaller cells of latewood from the previous year and the wide cells of the earlywood of the following year. No cells are formed during dormancy.
When redcedars experience optimal conditions they will grow quickly, putting on a wide ring. In non-optimal conditions, such as a lack of water or a short growing season, they can’t grow as much, and the ring ends up being much narrower.
Counting tree rings might seem like a simple process, but a very high level of care and knowledge is necessary to account for anomalies and inconsistencies while maintaining precision. For example, sometimes trees will put on what appears to be more than one ring per year, called a ‘false ring’, while other years a tree might not grow a ring at all. Eastern redcedar, tenacious as they are and living on some of the harshest sites in the region, are notoriously tricky, sometimes foregoing numerous rings in a row on one side of the tree while surging in growth on the other. This means that many samples need to be thoroughly studied and cross-analyzed before a chronology emerges. But once this process is finished, the chronology will be an exact annual record of tree growth for that area.
“When we say there was a bad drought in 1632, where a lot of cedars literally didn’t even have enough energy to put on a ring, we’re not talking about anything other than 1632,” says Larson. “The precision that’s available through these records is just unmatched.”
Once tree rings are situated within the chronology, they can be further analyzed for information. Depending on the site and region where trees grow, researchers can extrapolate data such as yearly growing conditions as well as rare, dramatic events like large fires, ice storms or particularly harsh winters. This study is specifically concerned with flash drought reconstruction and telling the historical climate story of the Great Plains region. The understanding of historical climate that this reconstruction aims to provide is far more relevant to everyday life than most people realize.
In particular, Larson and Riser mention farmers and insurance companies — it’s easy to ignore when things are going well, but unexpected climate events, like drought or fire, can bring about large-scale devastation.
“Knowing if a 10-year long drought happens every now and then is really good to know,” says Riser. “It will very likely happen again… you can handle a few years of drought, but can you absorb 10 years of crop failure? This data and record can help us prepare.”
Trees are the ideal tool for understanding climate history due to their long lifespan and detailed memory. Additionally, they are impartial observers of the world — tree rings don’t try to convince you of one thing or another, they simply demonstrate how hard or how easy it was to grow each year.
“Only a very few people alive now remember the dust bowl, but it was a formative drought for our nation’s history,” Larson says. “What about the droughts before then? What about the droughts over the past 500 years? Or 1000? What other events happened in the past that we can learn from to better plan for the future?”
“I would hope that [the results of the study] are used to help inform policies and human actions,” adds Riser.
From the information written into their rings to the insight that can be gleaned from observing how they grow and spread in our current landscape, redcedars have many important lessons to teach. When you are observing these old cedar trees, you are literally observing old growth forests that rival some western forests for age and longevity, which is not something we often expect in Iowa.
Perhaps the first step we can take is to acknowledge their wisdom and start paying attention to the expressions of the land.
If you know of a site you think harbors ancient cedars that could contribute to this study, consider contacting James (riserj@uwplatt.edu) or Evan (larsonev@uwplatt.edu) with photos and location!
See the TREES Lab in action through the video below, taken during a field day at Heritage Valley.